- Email: [email protected]
Micromachined thermal accelerometer
Sensors and Actuators A 103 (2003) 359±363
Micromachined thermal accelerometer F. Maillya,*, A. Giania, A. Martineza, R. Bonnota, P. Temple-Boyerb, A. Boyera a
Unite mixte de Recherche du CNRS no. 5507, Centre d'Electronique et de Micro-optoeÂlectronique de Montpellier, Universite Montpellier II, Place E. Bataillon, 34095 Montpellier, France b LAAS-CNRS, 7 rue du Colonel Roche, 31077 Toulouse Cedex 4, France Received 5 April 2002; received in revised form 11 August 2002; accepted 29 September 2002
Abstract The techniques of micromachining silicon are used for the manufacturing of a thermal accelerometer. This sensor requires no solid proof mass and has a low cost production. A heating resistor creates a symmetrical temperature pro®le and two temperature detectors are placed on both sides. When an acceleration is applied, the temperature pro®le becomes asymmetric and the two detectors measure the differential temperature. Platinum resistors deposited by electron beam evaporation on a SiNx membrane are used as heater and temperature sensors. This paper presents measurements of temperature pro®le and sensitivity according to the distance heater±detector, power supplied and room temperature. It shows that the sensitivity is proportional to the heating power and decreases when the room temperature increases. Experimental results in tiltmeter utilization and in a centrifuge are also presented. A 3 dB bandwidth of 20 Hz is measured applying a sinusoidal acceleration and the equivalent acceleration noise is found to be 0.3 mg RMS. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal sensor; Accelerometer; Tilt measurement
1. Introduction According to the fundamental principles of mechanics, acceleration corresponds to a relation between a force and a mass. The accelerometers use in general the direct measurement of a force (piezoelectric sensor, sensor with balance of couple or force) or the measurement of displacement or deformation of a proof mass. Thermal accelerometers with seismic mass have been studied by DauderstaÈdt et al. [1±3] but the mobility of the mass is the principal cause for the fragility of this type of sensor during very high accelerations such as those produced at the time of the ®ring of ballistic missiles. Therefore, thermal accelerometers without seismic mass were the subject of new studies since they differ from all the other types of accelerometers by the absence of seismic mass. This kind of sensor has been already reported in the literature [4±8] and the Fig. 1 presents its principle. A suspended heater creates a symmetric temperature pro®le in a hermetic chamber and two temperature detectors are symmetrically suspended on both sides of the heater. Without an acceleration, the two detectors have the same temperature, while when an acceleration is applied, a difference * Corresponding author. Tel.: 33-467143785. E-mail address: [email protected] (F. Mailly).
of temperature appears between the two detectors because of asymmetric heat transfer. Thermal resistance between the detector and the surrounding substrate must be as high as possible to limit the energy consumption. Thin ®lm resistors on a SiNx membrane, manufactured with silicon micromachining, make it possible to achieve this goal. We have chosen platinum for heater and detecting resistive material because it is the element that presents the physical properties most stable in time with a high temperature coef®cient of resistance (TCR) of 3:9 10 3 /8C for bulk material compared to the 2:2 10 3 /8C of polysilicon. Finally, a ``push±pull'' Wheatstone bridge permits to convert the resistance variations into a voltage output. 2. Microstructure design and fabrication Heater and thermal detectors are made of platinum thin ®lm on a low stress (s 0) silicon rich silicon nitride membrane SiNx [9]. The thicknesses of the SiNx and plaÊ , respectively. To improve tinum layers are 5000 and 3000 A adhesion, adhesion-promoting layers such as Ti or Cr are used but they tend to reduce the TCR [10]. Different methods of Pt deposition (ac sputtering, magnetron and electron beam evaporation) and post-annealing conditions
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 4 2 8 - 4
360
F. Mailly et al. / Sensors and Actuators A 103 (2003) 359±363
Fig. 1. Principle of the sensor.
were tested to improve the TCR and to prevent the layer from pealing off during KOH etching [11,12]. Best results have been obtained with electron beam evaporation and vacuum annealing: good adhesion is obtained even after 5 h in KOH etching solution, the electrical resistivity is about 15 mO cm and the TCR is 3:3 10 3 /8C. After Pt deposition the resistors are patterned by electron cyclotron resonance (ECR) etching. Then, the SiNx is etched by ECR to obtain resistors on SiNx bridges by KOH etching at 85 8C. Manufacturing stages are summarized on Fig. 2, Fig. 3 shows a scanning electron microscope (SEM) image of a sensor with three pairs of detectors and Fig. 4 presents its cross-section with its different dimensions.
Fig. 2. Manufacturing stages of accelerometer: SiNx deposition by LPCVD (1), Pt deposition (2), ECR etching of Pt (3), ECR etching of SiNx (4), KOH etching (5).
Fig. 3. SEM image of a sensor with three detectors pairs.
F. Mailly et al. / Sensors and Actuators A 103 (2003) 359±363
361
Fig. 4. Sensor cross-section.
3. Experimental results and discussions
(2)
with V, sensor response (V), E, supply bridge voltage (V), DR, resistance variation due to the acceleration (O) and R, resistance at 0 g (O). Therefore, the sensitivity is proportional to DR/R and its variations versus heater±detector distance and current heater are presented in Fig. 6. The sensitivity of the ®rst pair of detectors is very low while the sensitivities of the others are higher. The ®rst pair of detectors is too near to the heater and the temperature pro®le is little in¯uenced by the acceleration in this region. For this sensor, the optimum distance between the sensors and heater is about the half of the distance between the heater and the silicon chip. Fig. 7 shows the sensitivity versus ambient temperature for a constant heater current of 41 mA. The temperature dependence of the sensitivity is not linear and the temperature coef®cient is about 1% K 1 for a low temperature range. In comparison, a thermal accelerometer with a seismic mass [1±3] presents a linear temperature dependence and a temperature coef®cient of 0.12% K 1 while the thermal accelerometers without seismic mass, commercialized by the society MEMSIC, would use a gain adjustment of 0.9% K 1 to keep
Fig. 5. Temperature measurement of the heater and detectors vs. heater current.
Fig. 6. Sensitivity variations according to the heater±detector distance vs. current heater.
Detectors average temperatures have been measured using electrical resistance measurements for different heater power consumption: T
R T R0 aR0
(1)
with T, detector average temperature (8C), R(T), electrical resistance (O), R0, electrical resistance at 0 8C (O) and a, temperature coef®cient of resistance (8C 1). For the heating resistor, electrical resistance is deducted from Ohm's law: R T U/I. Fig. 5 presents experimental results versus heater current. We can observe the pro®le symmetry and a very high temperature decrease when the distance heater±detector increases. If the Wheatstone bridge is supplied with a constant voltage, the sensor response is given by: V
E DR 2 R
362
F. Mailly et al. / Sensors and Actuators A 103 (2003) 359±363
Fig. 7. Sensitivity vs. room temperature for a constant heater current of 41 mA.
the sensitivity within 5% of its room temperature value [14]. Leung et al. [6,7] have developed a simple model suggesting that the response of thermal accelerometers is linearly proportional to the Grashof number, Gr, given by:
Fig. 9. Electronically amplified (G 2000) sensor response in a centrifuge vs. acceleration (amplified sensitivity near 0 g: 116 mV g 1).
with a, acceleration (or earth gravity), r, gas density, b, gas coef®cient of expansion, DT, heater temperature, l, linear dimension and m, gas viscosity. For an ideal gas, the density and the coef®cient of expansion are linearly proportional to 1/T and the viscosity is proportional to T1/2. Therefore, it can explain the sensitivity decrease as ambient room temperature. Tilt measurements have been carried out by mounting the sensor on a vertical goniometer. The axis of rotation is horizontal and parallel to the resistors' axes. The sensor position is given by the angle y, which is the angle between the normal vector of the chip's surface and the earth gravity vector. The results are presented in Fig. 8: in these
conditions, the applied acceleration depends on the sinus of the tilt angle. For the measurement of higher accelerations, we have used a centrifuge and Fig. 9 presents the ampli®ed sensor response. It shows that the sensor response is not linear for higher accelerations: for 1 g, the differential temperature between the two detectors is typically about 2 8C when the heater temperature is 200 8C. Therefore, we should have a differential temperature higher than the heater temperature to get a linear response for very high accelerations. This is not possible and that is why we observe a saturation of the sensor response. For these sensors we have a relatively good linearity for a range about 0±3 g while a thermal accelerometer with a seismic mass presents a linear response in 0±0.4 g range [1]. Using sinusoidal acceleration, sensor 3 dB bandwidth was determined to be about 20 Hz (Fig. 10). Leung et al. [6,7] have reported the same response time for their device and the bandwidth of the MEMSIC sensor is also 25 or 30 Hz. Therefore, we think that gas properties and package volume are the critical parameters of sensor bandwidth.
Fig. 8. Resistor variations output vs. sensor tilt.
Fig. 10. Sensor response vs. sinusoidal acceleration frequency.
Gr
ar2 b DTl3 m2
(3)
F. Mailly et al. / Sensors and Actuators A 103 (2003) 359±363
Finally, the equivalent acceleration noise is found to be 0.3 mg RMS, which is equivalent to a differential temperature variation <10 3 K. For comparison, a platinum resistance used as temperature sensor in a Wheatstone bridge has a resolution of 2:6 10 4 K for a minimum measurement of DR/R 10 6 [13]. 4. Conclusion Temperature pro®le measurements on a thermal accelerometer have been investigated and have shown that the temperature rapidly decreases when the distance increases. The sensitivity was found to have a maximum if the distance heater±detector was about 300±500 mm. We have shown that the sensitivity is proportional to the heating power and decreases when the ambient temperature increases. Experimental results in tiltmeter utilization and in a centrifuge have shown that the sensors have a good linearity for a range about 0±3 g. Finally, a 3 dB bandwidth of 20 Hz was measured applying a sinusoidal acceleration and the equivalent acceleration noise was found to be 0.3 mg RMS. To increase the bandwidth, cavity volume should be reduced or another gas should be used because gas properties and package volume seem to be the critical parameters of sensor time response. References [1] U.A. DauderstaÈdt, P.H.S. de Vries, R. Hiratsuka, P.M. Sarro, Silicon accelerometer based on thermopiles, Sens. Actuators A 46±47 (1995) 201±204. [2] U.A. DauderstaÈdt, P.H.S. de Vries, R. Hiratsuka, J.G. Korvink, P.M. Sarro, H. Baltes, S. Middelhoek, Simulation aspects of a thermal accelerometer, Sens. Actuators A 55 (1996) 3±6. [3] U.A. DauderstaÈdt, P.M. Sarro, P.J. French, Temperature dependence and drift of a thermal accelerometer, Sens. Actuators A 66 (1998) 244±249. [4] V. Milanovic, E. Bowen, M.E. Zaghloul, N.H. Tea, J.S. Suehle, B. Payne, M. Gaitan, Micromachined convective accelerometers in standard integrated circuits technology, Appl. Phys. Lett. 76 (4) (2000) 508±510. [5] V. Milanovic, E. Bowen, N. Tea, J. Suehle, B. Payne, M. Zaghloul, M. Gaitan, Convection-based accelerometer and tilt sensor implemented in standard CMOS, in: Proceedings of the MEMS Symposia on International Mechanical Engineering and Exposition, Anaheim, CA, 1998, pp. 627±630. [6] A.M. Leung, J. Jones, E. Czyzewska, J. Chen, B. Woods, Micromachined accelerometer based on convection heat transfer, MEMS 98 (1998) 627±630. [7] A.M. Leung, J. Jones, E. Czyzewska, J. Chen, M. Pascal, Micromachined accelerometer with no proof mass, in: Proceedings of the Technical Digest of International Electron Device Meeting (IEDM'97), 1997, pp. 899±902. [8] S. Billat, H. Glosch, M. Kunze, F. Hedrich, J. Frech, J. Auber, H. Sandmaier, W. Wimmer, W. Lang, Micromachined inclinometer with high sensitivity and very good stability, Sens. Actuators A 97±98 (2002) 125±130.
363
[9] P. Temple-Boyer, C. Rossi, E. Saint-Etienne, E. Scheid, Residual stress in low pressure chemical vapor deposition SiNx films deposited from silane and ammonia, J. Vac. Sci. Technol. A 16 (4) (1998) 2003±2007. [10] J.-S. Lee, H.-D. Park, S.-M. Shin, J.-W. Park, Agglomeration phenomena of high temperature coefficient platinum films deposited by electron beam evaporation, J. Mater. Sci. Lett. 16 (1997) 1257±1259. [11] F. Mailly, A. Giani, R. Bonnot, P. Temple-Boyer, F. Pascal-Delannoy, A. Foucaran, A. Boyer, Anemometer with hot platinum thin film, Sens. Actuators A 94 (2001) 32±38. [12] A. Giani, F. Mailly, F. Pascal-Delannoy, A. Foucaran, A. Boyer, Investigation of Pt/Ti bilayer on SiNx/Si substrates for thermal sensor applications, J. Vac. Sci. Technol. A 20 (1) (2002) 112±116. [13] G. Asch et collaborateurs, Les Capteurs en Instrumentation Industrielle, 5th ed., ISBN 2 10 003773 0, Dunod, Paris, 1998. [14] R. Dao, Thermal accelerometers Temperature Compensation, MEMSIC Application note, 2002, http://www.memsic.com.
Biographies FreÂdeÂrick Mailly was born in Montmorency, France. He received the ``Doctorat'' degree in Electronics from Montpellier University in 1999. Since then, he has been preparing a PhD thesis on ``Fabrication of a thermal accelerometer'' in the Center of Electronics and MicroOptoelectronics of Montpellier where he works on material fabrication, electronic characterization and modelisation of silicon microsensors. Alain Giani was born in Arles, France. He received the PhD Degree in Electronics from Montpellier University in 1992. Since then, he works in the Center of Electronic and Micro-Optoelectronic of Montpellier University, where he is a specialist on vacuum deposition techniques. Presently, he is involved in thermal sensors for flow measurements and optoelectronic applications. Alexandre Martinez was born in AleÁs, France. He received the ``Doctorat'' degree in Electronics from Montpellier University in 2000. Since then, he has been preparing a PhD thesis on ``Modelisation and simulation of a thermal accelerometer'' in the Center of Electronics and MicroOptoelectronics of Montpellier where he works on modelisation, numerical simulation and optimization of silicon microsensors. Roger Bonnot was born in Paris, France. He obtained the DEST in Physics from CNAM. He worked from 1967 to 1988 in Reims University on fabrication of thin solid films for Electronic Microscopy. In 1988, he joined the Microelectronic Technology Center of Plasma Montpellier, Montpellier University. He works on plasma deposition, plasma etching and microstructure fabrication. Pierre Temple-Boyer was born on 25 October 1966, and received a degree in electronics engineering from the Ecole SupeÂrieure d'Electricite (SUPELEC-France) in 1990. He joined the LAAS-CNRS in 1992 and received the PhD degree from the Institut National des Sciences AppliqueÂes, Toulouse (INSAT-Toulouse) in 1995. Since then, as a researcher in LAAS-CNRS, he has been working on the development of new materials for microelectronics and Microsystems. Andre Boyer was born in Perpignan, France. He is Doctor ``es Sciences Physiques'' from Montpellier, University 1975. Since then, he works on the Center of Electronic and Micro-Optoelectronic of Montpellier, Montpellier University. He is a specialist in thermocouple temperature measurement, preparation and properties of thin solid films and ultrasonic method in Solid State Physics. Presently, he is involved in the fundamental studies of sensor phenomena and thermal transport processes in small structures.
Micromachined thermal accelerometer F. Maillya,*, A. Giania, A. Martineza, R. Bonnota, P. Temple-Boyerb, A. Boyera a
Unite mixte de Recherche du CNRS no. 5507, Centre d'Electronique et de Micro-optoeÂlectronique de Montpellier, Universite Montpellier II, Place E. Bataillon, 34095 Montpellier, France b LAAS-CNRS, 7 rue du Colonel Roche, 31077 Toulouse Cedex 4, France Received 5 April 2002; received in revised form 11 August 2002; accepted 29 September 2002
Abstract The techniques of micromachining silicon are used for the manufacturing of a thermal accelerometer. This sensor requires no solid proof mass and has a low cost production. A heating resistor creates a symmetrical temperature pro®le and two temperature detectors are placed on both sides. When an acceleration is applied, the temperature pro®le becomes asymmetric and the two detectors measure the differential temperature. Platinum resistors deposited by electron beam evaporation on a SiNx membrane are used as heater and temperature sensors. This paper presents measurements of temperature pro®le and sensitivity according to the distance heater±detector, power supplied and room temperature. It shows that the sensitivity is proportional to the heating power and decreases when the room temperature increases. Experimental results in tiltmeter utilization and in a centrifuge are also presented. A 3 dB bandwidth of 20 Hz is measured applying a sinusoidal acceleration and the equivalent acceleration noise is found to be 0.3 mg RMS. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal sensor; Accelerometer; Tilt measurement
1. Introduction According to the fundamental principles of mechanics, acceleration corresponds to a relation between a force and a mass. The accelerometers use in general the direct measurement of a force (piezoelectric sensor, sensor with balance of couple or force) or the measurement of displacement or deformation of a proof mass. Thermal accelerometers with seismic mass have been studied by DauderstaÈdt et al. [1±3] but the mobility of the mass is the principal cause for the fragility of this type of sensor during very high accelerations such as those produced at the time of the ®ring of ballistic missiles. Therefore, thermal accelerometers without seismic mass were the subject of new studies since they differ from all the other types of accelerometers by the absence of seismic mass. This kind of sensor has been already reported in the literature [4±8] and the Fig. 1 presents its principle. A suspended heater creates a symmetric temperature pro®le in a hermetic chamber and two temperature detectors are symmetrically suspended on both sides of the heater. Without an acceleration, the two detectors have the same temperature, while when an acceleration is applied, a difference * Corresponding author. Tel.: 33-467143785. E-mail address: [email protected] (F. Mailly).
of temperature appears between the two detectors because of asymmetric heat transfer. Thermal resistance between the detector and the surrounding substrate must be as high as possible to limit the energy consumption. Thin ®lm resistors on a SiNx membrane, manufactured with silicon micromachining, make it possible to achieve this goal. We have chosen platinum for heater and detecting resistive material because it is the element that presents the physical properties most stable in time with a high temperature coef®cient of resistance (TCR) of 3:9 10 3 /8C for bulk material compared to the 2:2 10 3 /8C of polysilicon. Finally, a ``push±pull'' Wheatstone bridge permits to convert the resistance variations into a voltage output. 2. Microstructure design and fabrication Heater and thermal detectors are made of platinum thin ®lm on a low stress (s 0) silicon rich silicon nitride membrane SiNx [9]. The thicknesses of the SiNx and plaÊ , respectively. To improve tinum layers are 5000 and 3000 A adhesion, adhesion-promoting layers such as Ti or Cr are used but they tend to reduce the TCR [10]. Different methods of Pt deposition (ac sputtering, magnetron and electron beam evaporation) and post-annealing conditions
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 4 2 8 - 4
360
F. Mailly et al. / Sensors and Actuators A 103 (2003) 359±363
Fig. 1. Principle of the sensor.
were tested to improve the TCR and to prevent the layer from pealing off during KOH etching [11,12]. Best results have been obtained with electron beam evaporation and vacuum annealing: good adhesion is obtained even after 5 h in KOH etching solution, the electrical resistivity is about 15 mO cm and the TCR is 3:3 10 3 /8C. After Pt deposition the resistors are patterned by electron cyclotron resonance (ECR) etching. Then, the SiNx is etched by ECR to obtain resistors on SiNx bridges by KOH etching at 85 8C. Manufacturing stages are summarized on Fig. 2, Fig. 3 shows a scanning electron microscope (SEM) image of a sensor with three pairs of detectors and Fig. 4 presents its cross-section with its different dimensions.
Fig. 2. Manufacturing stages of accelerometer: SiNx deposition by LPCVD (1), Pt deposition (2), ECR etching of Pt (3), ECR etching of SiNx (4), KOH etching (5).
Fig. 3. SEM image of a sensor with three detectors pairs.
F. Mailly et al. / Sensors and Actuators A 103 (2003) 359±363
361
Fig. 4. Sensor cross-section.
3. Experimental results and discussions
(2)
with V, sensor response (V), E, supply bridge voltage (V), DR, resistance variation due to the acceleration (O) and R, resistance at 0 g (O). Therefore, the sensitivity is proportional to DR/R and its variations versus heater±detector distance and current heater are presented in Fig. 6. The sensitivity of the ®rst pair of detectors is very low while the sensitivities of the others are higher. The ®rst pair of detectors is too near to the heater and the temperature pro®le is little in¯uenced by the acceleration in this region. For this sensor, the optimum distance between the sensors and heater is about the half of the distance between the heater and the silicon chip. Fig. 7 shows the sensitivity versus ambient temperature for a constant heater current of 41 mA. The temperature dependence of the sensitivity is not linear and the temperature coef®cient is about 1% K 1 for a low temperature range. In comparison, a thermal accelerometer with a seismic mass [1±3] presents a linear temperature dependence and a temperature coef®cient of 0.12% K 1 while the thermal accelerometers without seismic mass, commercialized by the society MEMSIC, would use a gain adjustment of 0.9% K 1 to keep
Fig. 5. Temperature measurement of the heater and detectors vs. heater current.
Fig. 6. Sensitivity variations according to the heater±detector distance vs. current heater.
Detectors average temperatures have been measured using electrical resistance measurements for different heater power consumption: T
R T R0 aR0
(1)
with T, detector average temperature (8C), R(T), electrical resistance (O), R0, electrical resistance at 0 8C (O) and a, temperature coef®cient of resistance (8C 1). For the heating resistor, electrical resistance is deducted from Ohm's law: R T U/I. Fig. 5 presents experimental results versus heater current. We can observe the pro®le symmetry and a very high temperature decrease when the distance heater±detector increases. If the Wheatstone bridge is supplied with a constant voltage, the sensor response is given by: V
E DR 2 R
362
F. Mailly et al. / Sensors and Actuators A 103 (2003) 359±363
Fig. 7. Sensitivity vs. room temperature for a constant heater current of 41 mA.
the sensitivity within 5% of its room temperature value [14]. Leung et al. [6,7] have developed a simple model suggesting that the response of thermal accelerometers is linearly proportional to the Grashof number, Gr, given by:
Fig. 9. Electronically amplified (G 2000) sensor response in a centrifuge vs. acceleration (amplified sensitivity near 0 g: 116 mV g 1).
with a, acceleration (or earth gravity), r, gas density, b, gas coef®cient of expansion, DT, heater temperature, l, linear dimension and m, gas viscosity. For an ideal gas, the density and the coef®cient of expansion are linearly proportional to 1/T and the viscosity is proportional to T1/2. Therefore, it can explain the sensitivity decrease as ambient room temperature. Tilt measurements have been carried out by mounting the sensor on a vertical goniometer. The axis of rotation is horizontal and parallel to the resistors' axes. The sensor position is given by the angle y, which is the angle between the normal vector of the chip's surface and the earth gravity vector. The results are presented in Fig. 8: in these
conditions, the applied acceleration depends on the sinus of the tilt angle. For the measurement of higher accelerations, we have used a centrifuge and Fig. 9 presents the ampli®ed sensor response. It shows that the sensor response is not linear for higher accelerations: for 1 g, the differential temperature between the two detectors is typically about 2 8C when the heater temperature is 200 8C. Therefore, we should have a differential temperature higher than the heater temperature to get a linear response for very high accelerations. This is not possible and that is why we observe a saturation of the sensor response. For these sensors we have a relatively good linearity for a range about 0±3 g while a thermal accelerometer with a seismic mass presents a linear response in 0±0.4 g range [1]. Using sinusoidal acceleration, sensor 3 dB bandwidth was determined to be about 20 Hz (Fig. 10). Leung et al. [6,7] have reported the same response time for their device and the bandwidth of the MEMSIC sensor is also 25 or 30 Hz. Therefore, we think that gas properties and package volume are the critical parameters of sensor bandwidth.
Fig. 8. Resistor variations output vs. sensor tilt.
Fig. 10. Sensor response vs. sinusoidal acceleration frequency.
Gr
ar2 b DTl3 m2
(3)
F. Mailly et al. / Sensors and Actuators A 103 (2003) 359±363
Finally, the equivalent acceleration noise is found to be 0.3 mg RMS, which is equivalent to a differential temperature variation <10 3 K. For comparison, a platinum resistance used as temperature sensor in a Wheatstone bridge has a resolution of 2:6 10 4 K for a minimum measurement of DR/R 10 6 [13]. 4. Conclusion Temperature pro®le measurements on a thermal accelerometer have been investigated and have shown that the temperature rapidly decreases when the distance increases. The sensitivity was found to have a maximum if the distance heater±detector was about 300±500 mm. We have shown that the sensitivity is proportional to the heating power and decreases when the ambient temperature increases. Experimental results in tiltmeter utilization and in a centrifuge have shown that the sensors have a good linearity for a range about 0±3 g. Finally, a 3 dB bandwidth of 20 Hz was measured applying a sinusoidal acceleration and the equivalent acceleration noise was found to be 0.3 mg RMS. To increase the bandwidth, cavity volume should be reduced or another gas should be used because gas properties and package volume seem to be the critical parameters of sensor time response. References [1] U.A. DauderstaÈdt, P.H.S. de Vries, R. Hiratsuka, P.M. Sarro, Silicon accelerometer based on thermopiles, Sens. Actuators A 46±47 (1995) 201±204. [2] U.A. DauderstaÈdt, P.H.S. de Vries, R. Hiratsuka, J.G. Korvink, P.M. Sarro, H. Baltes, S. Middelhoek, Simulation aspects of a thermal accelerometer, Sens. Actuators A 55 (1996) 3±6. [3] U.A. DauderstaÈdt, P.M. Sarro, P.J. French, Temperature dependence and drift of a thermal accelerometer, Sens. Actuators A 66 (1998) 244±249. [4] V. Milanovic, E. Bowen, M.E. Zaghloul, N.H. Tea, J.S. Suehle, B. Payne, M. Gaitan, Micromachined convective accelerometers in standard integrated circuits technology, Appl. Phys. Lett. 76 (4) (2000) 508±510. [5] V. Milanovic, E. Bowen, N. Tea, J. Suehle, B. Payne, M. Zaghloul, M. Gaitan, Convection-based accelerometer and tilt sensor implemented in standard CMOS, in: Proceedings of the MEMS Symposia on International Mechanical Engineering and Exposition, Anaheim, CA, 1998, pp. 627±630. [6] A.M. Leung, J. Jones, E. Czyzewska, J. Chen, B. Woods, Micromachined accelerometer based on convection heat transfer, MEMS 98 (1998) 627±630. [7] A.M. Leung, J. Jones, E. Czyzewska, J. Chen, M. Pascal, Micromachined accelerometer with no proof mass, in: Proceedings of the Technical Digest of International Electron Device Meeting (IEDM'97), 1997, pp. 899±902. [8] S. Billat, H. Glosch, M. Kunze, F. Hedrich, J. Frech, J. Auber, H. Sandmaier, W. Wimmer, W. Lang, Micromachined inclinometer with high sensitivity and very good stability, Sens. Actuators A 97±98 (2002) 125±130.
363
[9] P. Temple-Boyer, C. Rossi, E. Saint-Etienne, E. Scheid, Residual stress in low pressure chemical vapor deposition SiNx films deposited from silane and ammonia, J. Vac. Sci. Technol. A 16 (4) (1998) 2003±2007. [10] J.-S. Lee, H.-D. Park, S.-M. Shin, J.-W. Park, Agglomeration phenomena of high temperature coefficient platinum films deposited by electron beam evaporation, J. Mater. Sci. Lett. 16 (1997) 1257±1259. [11] F. Mailly, A. Giani, R. Bonnot, P. Temple-Boyer, F. Pascal-Delannoy, A. Foucaran, A. Boyer, Anemometer with hot platinum thin film, Sens. Actuators A 94 (2001) 32±38. [12] A. Giani, F. Mailly, F. Pascal-Delannoy, A. Foucaran, A. Boyer, Investigation of Pt/Ti bilayer on SiNx/Si substrates for thermal sensor applications, J. Vac. Sci. Technol. A 20 (1) (2002) 112±116. [13] G. Asch et collaborateurs, Les Capteurs en Instrumentation Industrielle, 5th ed., ISBN 2 10 003773 0, Dunod, Paris, 1998. [14] R. Dao, Thermal accelerometers Temperature Compensation, MEMSIC Application note, 2002, http://www.memsic.com.
Biographies FreÂdeÂrick Mailly was born in Montmorency, France. He received the ``Doctorat'' degree in Electronics from Montpellier University in 1999. Since then, he has been preparing a PhD thesis on ``Fabrication of a thermal accelerometer'' in the Center of Electronics and MicroOptoelectronics of Montpellier where he works on material fabrication, electronic characterization and modelisation of silicon microsensors. Alain Giani was born in Arles, France. He received the PhD Degree in Electronics from Montpellier University in 1992. Since then, he works in the Center of Electronic and Micro-Optoelectronic of Montpellier University, where he is a specialist on vacuum deposition techniques. Presently, he is involved in thermal sensors for flow measurements and optoelectronic applications. Alexandre Martinez was born in AleÁs, France. He received the ``Doctorat'' degree in Electronics from Montpellier University in 2000. Since then, he has been preparing a PhD thesis on ``Modelisation and simulation of a thermal accelerometer'' in the Center of Electronics and MicroOptoelectronics of Montpellier where he works on modelisation, numerical simulation and optimization of silicon microsensors. Roger Bonnot was born in Paris, France. He obtained the DEST in Physics from CNAM. He worked from 1967 to 1988 in Reims University on fabrication of thin solid films for Electronic Microscopy. In 1988, he joined the Microelectronic Technology Center of Plasma Montpellier, Montpellier University. He works on plasma deposition, plasma etching and microstructure fabrication. Pierre Temple-Boyer was born on 25 October 1966, and received a degree in electronics engineering from the Ecole SupeÂrieure d'Electricite (SUPELEC-France) in 1990. He joined the LAAS-CNRS in 1992 and received the PhD degree from the Institut National des Sciences AppliqueÂes, Toulouse (INSAT-Toulouse) in 1995. Since then, as a researcher in LAAS-CNRS, he has been working on the development of new materials for microelectronics and Microsystems. Andre Boyer was born in Perpignan, France. He is Doctor ``es Sciences Physiques'' from Montpellier, University 1975. Since then, he works on the Center of Electronic and Micro-Optoelectronic of Montpellier, Montpellier University. He is a specialist in thermocouple temperature measurement, preparation and properties of thin solid films and ultrasonic method in Solid State Physics. Presently, he is involved in the fundamental studies of sensor phenomena and thermal transport processes in small structures.